The sites of action of GH in the human infant remain unclear; recent evidence in animals suggests direct actions on growth plate and other tissues. We have used a monoclonal antibody recognizing the human GH receptor to visually identify and localize GH receptors in the human infant growth plate. Sternochondral cartilage was obtained at postmortem from infants dying of sudden infant death (n = 20), and either decalcified, fixed, and cut into longitudinal sections or digested with collagenase for monolayer culture of chondrocytes. Sections of cultured chondrocytes were stained immunocytochemically with a monoclonal antibody recognizing human GH receptor (MAb 263), using an avidin-biotin system. Sternochondral cartilage was also obtained at operation from adolescents undergoing sternochondroplasty. In infant tissue, GH receptor was identified in sections in chondrocytes of the proliferative and hypertrophic layers, in perichondrium, in osteocytes in new bone, and in hemopoietic precursor cells in marrow. Cultured chondrocytes showed heterogeneous staining for GH receptor. With prolonged culture from 5-8 days, the pattern of staining changed from individual cells to groups of cells. [125I]Human (h)GH showed specific binding to chondrocyte monolayer (0.6 +/- 0.3%), confirmed visually on emulsion autoradiography. In support of specificity of MAb263, it was able to displace [125I]hGH from monolayers by 35%. Adolescent cultured chondrocytes failed to demonstrate specific binding of [125I]hGH. We conclude that GH receptors are widely distributed in a range of mesenchyme cells in the human infant growth plate, including bone and hemopoietic precursors. The expression of these receptors appears to be maturation dependent in both intact tissue and culture, while they may no longer be expressed after the peak growth phase of puberty.